In the field of optical communications, Dense Wavelength Division Multiplexing (DWDM) in which signals are multiplexed by being carried on a plurality of different wavelengths are generally used in order to handle added communications capacity. In such a field, optical wavelength conversion between the respective nodes of the network will be indispensable in the future for the effective utilization of the limited wavelength resources of the respective nodes. In the past, furthermore, wavelength multiplex communications have been performed only in the C band; however, there are prospects for the use of other wavelength bands as a result of the increase in communication capacity. Accordingly, a need has arisen for inter-band wavelength conversion between the respective nodes of such networks.
In conventional wavelength conversion techniques, wavelength conversion is performed after the optical signal is temporarily converted into an electrical signal; then, the signal is again converted into an optical signal. In such a method, however, the following problem arises: namely, the conversion speed is slow, and a conversion device is required for each wavelength. Accordingly, this method is disadvantageous in terms of integration and cost.
In recent years, on the other hand, there has been active research concerning optical devices that are capable of a direct wavelength conversion from light to light. Such a method has extremely great merit in that conversion can be accomplished without losing phase information of the light, and in that a simple system can be constructed that is capable of extremely high-speed conversion compared to systems in which wavelength conversion is performed via electrical signals.
Currently, a number of methods for the direct conversion of light have been proposed. First, a wavelength conversion method using mutual gain modulation by means of a compound semiconductor amplifier has been considered. In the case of this method, however, the following problem arises: namely, the wavelength conversion band is limited to the interior portion of the C band within the gain range of the semiconductor, so that inter-band wavelength conversion is impossible.
Furthermore, a method using nondegenerate four-wave light mixing in an optical fiber has also been proposed. In this method, wavelength conversion in the optical communication wavelength band can be accomplished by nondegenerate four-wave light mixing utilizing the third-order nonlinearity of the gain medium. In this method, however, an optical fiber with a length of several tens to several hundreds of meters is necessary in order to improve the conversion efficiency. The possibility that the phase matching conditions will differ from place to place in such a long fiber is large, so that there are problems in terms of wavelength stability. Moreover, the following problem also arises: namely, since the wavelength bandwidth that can be converted is inversely proportional to the length of the fiber, the conversion band is limited.
Moreover, a method using a quasi-phase matching nonlinear optical element has also been proposed. The advantage of wavelength conversion using a quasi-phase matching nonlinear optical element is that the wavelength conversion of numerous wavelengths at one time can be accomplished without noise in a broad wavelength band. Accordingly, at the current point in time, the direct wavelength conversion of light using a quasi-phase matching nonlinear optical element would appear to be the most powerful method in Dense Wavelength Division Multiplexing (DWDM).
A quasi-phase matching nonlinear optical element will be described below. The generation of light of different frequencies, e.g., second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), or the like, can be accomplished by causing laser light to be incident on a second-order nonlinear optical medium that does not have the center symmetry.
However, in order to allow high-efficiency wavelength conversion that can stand up to practical use, it is necessary that the phases of the respective wavelengths satisfy certain matching conditions. Methods for causing these matching conditions to be satisfied include a method utilizing the birefringence of a crystal, and a method in which the sign of the nonlinear constant of a crystal is periodically inverted. The latter method is called quasi-phase matching (QPM).
Difference frequency generation is a technique in which light of a frequency ω1 and light of a frequency ω2 are caused to be incident, and are converted into a difference frequency ω3(=ω1−ω2). In a case where the nonlinear medium has a nonlinear-constant-inverted structure with a period of Λ, it is necessary to satisfy the quasi-phase matching condition determined by the following equation:β1−β2−β3−2πm/Λ=0Here, m is an integer, β1 and β2 are respectively the propagation constants of the light of frequency ω1 and light of frequency ω2 in the nonlinear medium, and β3 is the propagation constant of the light of frequency ω3 in the nonlinear medium.
A method in which a high electric field is applied to a ferroelectric substance which has spontaneous polarization, so that a periodic polarization inversion is performed, has been used as a method for inverting the nonlinear constants. Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3), etc., are known as typical examples of such substances.
Wavelength conversion in optical communications can be accomplished by difference frequency generation using this quasi-phase matching nonlinear optical element. For example, wavelength conversion in the vicinity of a wavelength of 1.5 μm used in optical communications is accomplished as described in a paper by C. Q. Xu, et al., titled “Wavelength conversions 1.5 m by difference frequency generation in periodically domain inverted LiNbO3 channel wave guides” (Appl. Phys. Lett. Vol. 63 (1993), pp. 1170–1172). Furthermore, there has also been extensive research on inter-band optical wavelength conversions.
Wavelength conversion in optical communications utilizing difference light prior to wavelength conversion are respectively designated as λin an ωin. The wavelength and frequency of the light following wavelength conversion are respectively designated as λout and ωout. Furthermore, another type of light that is required for difference frequency generation is called “pump light” (control light); the wavelength and frequency of this light are respectively designated as λpump and ωpump. A polarization inversion structure with a period of Λ is built in so that the phase matching conditions are satisfied by ωout=ωpump−ωin.
For example, in cases where a C-L band conversion is performed using lithium niobate, if the wavelength λpump of the pump light is set so that λpump=0.785 μm, then λin=1.53 to 1.57 μm, λout=1.57 to 1.61 μm, and the period Λ is approximately 19.5 μm. In difference frequency generation, in a case where the wavelength is shown in the horizontal axis and the intensity of the light is shown on the vertical axis as is shown in FIG. 1, the wavelength prior to conversion and the wavelength following conversion form mirror images about the axis of the pump light wavelength. Specifically, in a case where the wavelengths prior to conversion are λ1, λ2, . . . , λN, the wavelengths following conversion are respectively λ1′, λ2′, . . . , λN′, λn′, and in a case where the wavelengths prior to conversion are λ1′, λ2′, . . . , λN′, the wavelengths following conversion are respectively λ1, λ2, . . . , λN. Accordingly, the wavelength conversion of numerous wavelengths at one time can be accomplished without noise, and without the signals prior to wavelength conversion interfering with each other following conversion.
However, the following problems arise in a quasi-phase matching nonlinear optical element using a ferroelectric crystal. Namely, in the case of lithium niobate and lithium tantalate, temporal damage in the refractive index caused by the photorefractive effect (optical damage) is a problem. In this effect, the carrier is excited and diffused from impurities that are admixed in the crystal growth stage, so that a non-uniform distribution is produced. As a result, an internal electric field is generated, and a variation in the refractive index is generated via the electro-optical effect. This light-induced variation in the refractive index is also called optical damage; in a quasi-phase matching device, this variation causes a deviation from the phase matching conditions, and therefore causes a drop in conversion efficiency. Accordingly, this is an important factor limiting the performance.
A method in which the crystal temperature is maintained at 100° C. or higher is known as a method for reducing optical damage. In order to accomplish this, it is necessary to control the temperature by means of a Peltier element, etc. However, this method suffers from numerous problems, e.g., a device for heat radiation is necessary, and simple system cannot be constructed. Accordingly, this method is not suitable for use in practical optical devices.
On the other hand, crystals that are relatively resistant to optical damage, e.g., MgO-doped crystals and stoichiometric crystals, are being developed. In such materials, however, there are problems in terms of quality, availability, cost, difficulty of polarization inversion and the like, so that these materials involve problems in terms of practical use at the present time.
Furthermore, ordinary lithium niobate and the like are mass-produced as materials for use in high-frequency electrical signal filters; however, a higher-quality crystal (optical grade crystal) is required for use in optical applications. Such optical grade crystals can only be produced in small quantities; accordingly, there are problems in terms of cost, availability, etc.
Furthermore, strict conditions with respect to the ambient temperature are also a problem in lithium niobate and lithium tantalate. When the ambient temperature varies, the refractive index of the crystal also varies; as a result, the phase matching conditions cannot be satisfied, so that the wavelength conversion efficiency shows an abrupt decrease. In the case of lithium niobate, for example, a variation of a few degrees in the ambient temperature results in a deviation from the phase matching conditions. Accordingly, temperature control is required in order to maintain the temperature of the crystal within a few degrees even if the ambient temperature should vary. Consequently, the construction of the system is complicated.
Furthermore, these elements also suffer from problems in terms of the ability to couple with quartz type optical fibers. Currently, quartz type optical fibers are widely used as a medium for guiding light in optical communications. Moreover, optical waveguide structures based on the proton exchange method or the like have been manufactured and used for the purpose of increasing the wavelength conversion efficiency by a strong shutting in of the light in a polarization inversion element consisting of such lithium niobate or the like. In this case, the mode diameter of the optical waveguide is approximately 4 μm, while the mode diameter of a quartz type fiber is approximately 10 μm. Accordingly, it is impossible to achieve complete matching of both optical modes, so that a coupling loss of approximately 2 dB is inevitably generated when the optical waveguide and optical fiber are coupled.
The present invention was devised in light of such circumstances; the object of the present invention is to provide a wavelength converter for use in optical communications which has little problem of optical damage, which can be used in a broad range of temperatures, and which also shows good coupling characteristics with quartz type optical fibers.